1. Field of the Invention
The present invention relates to a method and apparatus for removing artifact from physiological signals, and more specifically, to using a heart rate value that is determined from a first physiological signal that is acquired independently from a second physiological signal, for controllably bandpass filtering the second physiological signal.
In a system having sensors for acquiring both electrocardiogram (ECG) and pulse oximetry (SpO2) signals, the ECG signal is used to controllably bandpass filter the red and infrared signals acquired by the pulse oximeter. As a result of the invention, more accurate measurement of blood oxygenation and pulse rate is achieved by the oximeter, and the number of false alarms due to erroneous measurements is greatly reduced.
2. Description of the Prior Art
As well known by those of ordinary skill in the art, a pulse oximeter measures arterial blood oxygen saturation and pulse rate using a sensor containing two LED's and a photodiode detector, which is applied directly to a well perfused part of a patient, such as at a finger or ear. The LED's of the sensor apply radiation of two different wavelenghts, commonly red and infrared, to the patient, and the photodiode detector responsive to red and infrared light develops red and infrared electrical signals that are affected, via transmission or reflection, by the patient's blood flow in the area between the two LED's and photodiode detector. The greater the oxygenation of the blood, the less of the emitted red light is detected, due to greater absorption of the red light by the patient's blood. Consequently, the acquired red and infrared signals can be processed to develop a measurement indicative of the blood oxygenation. Additionally, by processing of the pulsatile component of the acquired light signals, a measurment of the pulse rate of the patient can also be developed. As well known, such processing for determining blood oxygenation is based on the ratio of the AC and DC components of the red light compared to the infrared light, such as: ##EQU1## The resultant value is applied to an experimentally-determined reference table to provide the final determination of the acquired measurement of the blood oxygenation.
As well known, the blood oxygenation and pulse rate measurements made from optically acquired signals are highly prone to inaccuracies due to artifacts. Such artifacts typically result from electrical interference (lights, electro-surgical and other electrical equipment near the patient), patient movement (causing a relative movement between the LED's and detector of the sensor, or even worse, the sudden admission of room light into area of the photodiode detector), as well as the fact that the AC component of the acquired signals (which results from the pulsatile characteristic of the blood), is very small, typically on the order of only 1%-5% of the DC value of the acquired signals. Consequently, such artifacts are extremely detrimental to accurate pulse oximetry measurements, and furthermore, can easily lead to the disturbing problem of false alarms.
U.S. Pat. No. 4,955,379 entitled MOTION ARTEFACT REJECTION SYSTEM FOR PULSE OXIMETERS, issued Sep. 11, 1990, discloses the a band-pass filtering (BPF) technique for removing noise artifacts from pulse oximetry signals. More specifically, the AC components of each of the acquired red and infrared signals is initially filtered by a BPF that is broadly tuned to the expected heart rate frequency. The output of the BPF is applied to a frequency determining circuit, whose output is then used to cause the BPF to track the frequency determined by the frequency determining circuit. The theory of this technique is that most of the energy (and information) in the AC signal is contained in at the fundamental frequency, and since the fundamental frequency should be the pulse rate, the frequency determining circuit will determine the pulse rate as the fundamental frequency and control the BPF to exclude all other frequencies, along with artifacts. Unfortunately, it is quite possible that the fundamental frequency determined by the frequency determining circuit may in fact be an artifact signal, such as one that is generated by electrical equipment, causing the oximeter to process the artifact signal and report erroneous information. Consequently, this technique is undesirable.
U.S. Pat. No. 4,928,692 entitled METHOD AND APPARATUS FOR DETECTING OPTICAL PULSES, issued May, 29, 1990, discloses a technique wherein the R-wave portion of a patient's ECG waveform is correlated in time with the optical signals acquired by a pulse oximeter. The correlation is used to develop an enabling signal for processing of the acquired optical signals by the oximeter. The theory is that since the pulse component of the optical signals contain the information, and the occurrence of the optical pulses can be predicted to follow an ECG R-wave by a certain amount, selective timing of oximeter enablement will prevent artifact from being admitted into the oximeter and erroneously processed. Unfortunately, since artifacts can occur at any time, and in general are not in any way correlated so as to have any relation to occurrence of an ECG R-wave, this technique is undesirable.